My first question is if one can use one email gateway to filter incoming emails for SPAM and viruses with all bells and whistles like this tutorial seems to do for MULTIPLE MTAs.
So what I mean is, can I have one Email Gateway filtering incoming emails then forwarding the emails for different domains to different servers? Say I have multiple small VPS machines which cannot run their own mail filtering due to resource limitations so I'd like to have one gateway handle
incoming mails for them while they will continue to work as outgoing mail servers but only accept incoming emails from the gateway?

The tutorial is for Ubuntu 12.04 as the title says, so you have the exact Ubuntu version (which even contains the date in its name) were this instruction apply to. So they are valid from the day Ubuntu 12.04 got released for Ubuntu 12.04 and they will work in the exact same way for Ubuntu 12.04 in 100 years. So the age of a tutorial does not matter at all as letters neither gets better nor worse when they get older. What matters is for which exact OS version it has been written and the OS version is Ubuntu 12.04 and that's mentioned in the title of the guide, so you can't miss that.This means that this tutorial is and has been tested for Ubuntu 12.04, it might be that it works in newer versions of course and its worth a try.